The subject of the present invention is a process and an apparatus for the continuous coating of at least one metal strip with a thin fluid film of crosslinkable polymer containing no unreactive solvent or diluent.
Thermally crosslinkable polymers such as, for example, thermosetting polymers, or physically crosslinkable polymers such as, for example, photocurable polymers, are known.
There is a wide variety of thermosetting organic coatings which are continuously applied to metal substrates.
In most cases, these are complex formulations which combine, in a solvent or aqueous medium, a system of prepolymer functional organic binders, a crosslinking system and additives such as pigments or fillers, and various formulation adjuvants.
Various processes are also known for applying a thermoplastic or thermosetting organic coating to a bare or coated metal strip.
The application of organic coatings such as, for example, liquid paints or varnishes is usually carried out by roller coating these liquid coatings in the state of a solution or of a dispersion in an aqueous or solvent medium.
To do this, the liquid coating is deposited on a metal strip by producing the solution or dispersion using a system comprising two or three rollers and by transferring some or all of this liquid coating thus predosed onto an applicator roller in contact with the surface of the metal strip to be coated.
The transfer is performed either by friction of the applicator roller on the metal strip, the two surfaces in contact running in opposite directions, or by contact in the same direction.
An advantageous trend in the technology of continuous application of crosslinkable polymer coatings, such as thermosetting paints or varnishes for example, to a metal strip consists in depositing this coating without the use of a solvent or a diluent.
Several alternatives have been proposed for producing and applying organic coatings without the use of an unreactive solvent or diluent.
Thus, to produce thin coatings of viscous organic products, another technique consists in extruding the organic coating in the fluid state and in applying this coating to a substrate by coating or lamination.
The extrusion-coating of a thin organic coating is widely practised, particularly with thermoplastic polymers on flexible surfaces, such as paper, plastic films, textiles or even thin metal substrates such as packaging metals.
The coating is applied in the melt state by means of a rigid sheet die or of a nozzle positioned in direct contact with the substrate.
The pressure exerted by the die on the substrate comes from the viscosity of the melt. Thus, the possibilities of correcting flatness defects of the substrate by pressing the latter against the back-up roll are very limited.
This extrusion-coating technique requires strict parallelism between the edges of the die and the substrate, and this substrate must be either perfectly plane or be deformable in order to allow a thin coating of uniform thickness to be formed.
This is because the thickness of the material deposited is controlled by the gap and the pressure between the die and the substrate, which requires strict parallelism between these two elements when it is desired to apply coatings with a very small thickness.
This condition cannot be achieved in the case of steel strip having a thickness of between 0.3 and 2 mm, which is too rigid and has a flatness or thickness uniformity which is insufficient to allow such an accurate adjustment of the gap between the die and the substrate, particularly in the case of wide strip.
The technique of extrusion lamination of a uniform layer of fluid coating on a substrate uses the drawing beneath the die of a fluid sheet at the exit of a sheet die, this sheet then being pressed against the substrate with the aid, for example, of a cold roller or of a rotating bar, or else by an air knife or an electrostatic field.
In this case, the thickness of the fluid sheet is controlled by the flow rate of the material in the die section and by the speed of the substrate.
Should the fluid sheet stick on the pressing roller, the sheet would then separate into two parts in its thickness, one part being applied to the substrate and the other part remaining applied to the roll. This separation of the sheet therefore means that transfer is not complete and the coating obtained on the substrate has neither a satisfactory surface appearance nor a uniform thickness.
In order to prevent the fluid sheet from sticking on the pressing roller, the latter must have a perfectly smooth and cooled surface.
The pressing pressure must however be low enough to prevent the formation of a calendering bead and consequently, this mode of transfer does not make it possible to compensate for any thickness variations and discrepancies in flatness in the case of a rigid substrate.
This technique of applying the coating with the formation of a free strand at the exit of the extrusion die makes it possible to avoid the problems of coupling between the die and the rigid substrate, but it causes application instabilities if the length of the free strand fluctuates and it is difficult to use with thermosetting systems having a viscosity of less than 2000 Paxc2x7s because of the difficulties in achieving uniform drawing and good lamination.
In general, in the various known techniques mentioned above, the continuous application of a thin organic coating to metal substrates is carried out with low contact pressures, insufficient to allow production of a thin uniform coating applied homogeneously to rigid substrates which may have flatness and thickness-heterogeneity discrepancies.
These various application techniques do not make it possible to compensate for the variations in thickness of the metal substrate, which variations consequently cause unacceptable fluctuations in the thickness of the coating, especially if the substrate is formed by a metal strip which exhibits significant surface roughness and/or corrugations of amplitude equal to or greater than the thickness of the coating to be produced on the said metal strip.
Moreover, these various application techniques do not make it possible to allow for variations in the width of the substrate nor variations in the transverse positioning of this substrate, so that the coating cannot be deposited uniformly over the entire width of the substrate.
Finally, during application of the coating, air microbubbles may be trapped between the coating and the substrate, which is to the detriment of homogeneous application and to the surface appearance of this coating.
Thus, a continuous application of a coating of crosslinkable polymer with a small and uniform thickness on a metal strip poses problems because this metal strip has flatness and thickness defects together with significant roughness and/or corrugations of amplitude equal to or greater than the thickness of the coating film to be deposited on the said strip, even when this strip is pressed with a high force on a uniform roll.
In addition, the various techniques used hitherto do not make it possible to apply, to a metal strip, a thin coating of crosslinkable polymer containing no unreactive solvent or diluent meeting two contradictory requirements, namely hardness and deformability.
This is because, after crosslinking the polymer coating must be hard enough while still being deformable in order to allow the sheet thus coated to be formed without a coating undergoing degradation or separation.
Now, it is known that increasing the molecular mass of the crosslinkable precursers of the polymer is highly favourable to obtaining a final coating which is both hard and deformable.
However, increasing the molecular mass of the precursors has a very unfavourable effect on the viscosity of a polymer containing no unreactive solvent or diluent, thereby impairing the ease of transfer and of application of the sheet, in the uncrosslinked melt state, to the metal strip.
The object of the invention is to avoid these drawbacks by providing a process and an apparatus for the continuous coating of at least one metal strip with a thin fluid film of crosslinkable polymer containing no unreactive solvent or diluent, and the softening temperature of which is greater than 50xc2x0 C., making it possible to obtain a coating of uniform thickness of a few microns to a few tens of microns, applied homogeneously to this strip, while preventing air microbubbles from being trapped between the film and the metal strip and obviating discrepancies in the flatness and roughness of this strip as well as allowing application on part or all of the coating, despite the fluctuations in width and transverse positioning of this strip.
The subject of the invention is therefore a process for the continuous coating of at least one metal strip with a fluid film of crosslinkable polymer containing no unreactive solvent or diluent and the softening temperature of which is greater than 50xc2x0 C., the said film having a thickness of less than that of the metal strip, characterized in that:
the metal strip is made to run continuously over at least one back-up roll;
a sheet of the said crosslinkable polymer is formed, on a roll and by forced flow at a temperature greater than the softening temperature of the crosslinkable polymer, the said polymer being crosslinkable in the melt state and having a viscosity greater than 10 Paxc2x7s under the conditions of formation of the said sheet, the temperature of formation of this sheet being lower than the crosslinking onset temperature of the crosslinkable polymer and the said roll being driven in rotation in the opposite direction to the direction in which the metal strip runs;
the said film of crosslinkable polymer is formed from the said sheet;
the film is completely transferred, thicknesswise, onto the metal strip; and
between the region of formation of the sheet on the roll and the region of application of the film to the metal strip, the crosslinkable polymer is thermally conditioned using suitable methods for lowering the viscosity of this crosslinkable polymer to a value less than the said viscosity measured under the conditions of the said forced flow.
The subject of the invention is also an apparatus for the continuous coating of at least one metal strip with a fluid film of crosslinkable polymer containing no unreactive solvent or diluent and the softening temperature of which is greater than 50xc2x0 C, the said film having a thickness of less than that of the metal strip, characterized in that it comprises:
means for continuously driving the metal strip;
at least one back-up roll supporting the metal strip;
means for forming, on a roll and by forced flow at a temperature greater than the softening temperature of the crosslinkable polymer, a sheet of the said polymer crosslinkable in the melt state and having a viscosity greater than 10 Paxc2x7s under the conditions of formation of the said sheet, the temperature of formation of this sheet being lower than the crosslinking onset temperature of the crosslinkable polymer and the said roll being driven in rotation in the opposite direction to the direction in which the metal strip runs;
means for forming, from the said sheet, the said film of crosslinkable polymer and for completely transferring, thicknesswise, this film onto the metal strip;
and, between the region of formation of the sheet on the roll and the region of application of the film to the metal strip, means for thermally conditioning the crosslinkable polymer using suitable methods for lowering the viscosity of this crosslinkable polymer to a value of less than the said viscosity measured under the conditions of the said forced flow.